readme: 'cargo llvm-cov' does not run doc-tests

[santhosh-tekuri]

readme suggests to use cargo llvm-cov instead of cargo test

but there is one difference cargo llvm-cov does not run doc-tests.
I think this should be mentioned explicitly in readme.

BTW, I replaced cargo test with cargo llvm-cov in my CI, and wondering why it is not showing any errors in doc tests.

[taiki-e]

readme says:

Support for doc tests. (this is currently optional and requires nightly, see #2 for more)

[santhosh-tekuri]

I saw it. But as a first time user, I jumped directly to Usage section. I guess most first time users do the same and don't realise this .

[taiki-e]

Ok, I added a description about doctests in the "Usage" section as well. Thanks for the feedback.